The present invention relates to optical fiber sensors and, more particularly, to a multi-mode optical fiber sensor and method for detecting changes in inter-mode interference patterns in response to external environmental perturbations.
Optical energy transmitted through the core of an optical fiber, either a single or multi-mode core, is affected by physical perturbations of the fiber. Typically, the physical perturbation will alter the index of refraction of the core material as well as the differential indices between the cladding and the core in such a way that the optical energy transmitted through the core is changed The physical perturbation can be caused by tension- or compression-induced strain as well as strain induced by bending the fiber about a small radius (i.e., micro-bending) or large radius bending (i.e., macro-bending). Accordingly, an optical fiber can be used as a sensor to measure a physical parameter by correlating changes in the output energy with the environmental perturbations.
The energy output from the sensing fiber can be analyzed, for example, in terms of quantitative changes in intensity, wavelength, or polarization states. In a more sophisticated context, the output light can be interferometrically compared against a reference source to provide an interferometer pattern that can be empirically correlated with the fiber-perturbing parameter. In the interferometric context, e.g., a Mach-Zender interferometer, coherent source light is passed through reference and sensing fibers with the light from the two paths combined to form an interferometric pattern that is analyzed to provide information that is functionally related to an external perturbation affecting the sensing fiber path.
Optical fiber sensing systems have a number of applications, especially in the aerospace field. For example, the wing or tail section of an aircraft can be `fibered` and the output light analyzed to determine stress levels in the structure as well as the presence of pre-fracture conditions or the presence of fractures caused by fatigue or ballistic impact. In addition, optical fiber sensing systems have the advantage of an exceptionally wide band-width so that the fiber can serve both a sensing function as well as data transmission function.